Eradication of Feral Goats from Small Islands

Eradication of feral goats from small islands

Kath Daly

Concern about the effects of feral goats on Aldabra atoll—a UNESCO World Heritage Site—led to a search for solutions. The search highlighted the inaccessibility of information on feral goat eradication campaigns. As a result the International Council for Bird Preservation and the FFPS, through its Oryx 100% Fund, commissioned a report to bring together and assess known techniques and potential strategies and to consider their implications for conservation practice. This article is a shortened version of that report.* Destructive effects of feral goats and demands for caparum yet discovered has had all flowers bitten their eradication are recurring themes in island off by feral goats (Lucas and Synge, 1978). The conservation literature (Atkinson, 1964; vine Canavalia kauensis was described only after Holdgate, 1967; Sykes, 1969; Wardle et al, the erection of a goat-proof exclosure, which 1973; Bourne, 1975; Mueller-Dombois and allowed it to germinate and survive (Mueller- Spatz, 1975; Pickard, 1976; Bullock, 1977; Dombois and Spatz, 1975). Coblentz, 1978; Newing etal, 1984; Calvopina, 1985; Coblentz and van Vuren, 1987). Examples Most feral goat impact on fauna is indirect, via of positive ecological effects of feral goats are habitat degradation. Other factors are often in- hard to find. Some argue that this is partly a re- volved, making this impact hard to quantify. On flection of research bias, but there is no disputing Lord Howe Island feral goats, together with pigs, that 'feral Caprinae have the potential for caused dramatic changes in habitat, which are ecological destruction' (Brooke, 1984) and that believed to have brought the island's woodhen, 'they are implicated in habitat destruction and Gallirallus sylvestris, close to extinction (Fullager the alteration of species composition of many and Disney, 1975). There are several reported sensitive insular ecosystems' (Howard and cases of direct competition for food between Marsh, 1984). feral goats and native island species. These include the Marquesas pigeon Ducula galeata (M. Feral goats eat seedlings, peel bark and even Fowler, in litt.), the New Zealand kokako Cal- climb trees to reach canopy leaves. They often laeas cinerea (Leathwick et al., 1983), the browse native species in preference to intro- Galapagos giant tortoise Geochelone elephantus duced ones (Baker and Reeser, 1972). Consequ- (McFarland et al, 1974), and the Aldabra giant ences include virtual destruction of a complete land tortoise Geochelone gigantea (Coblentz and forest (Bullock and North, 1985), restriction of van Vuren, 1987). regeneration (Parkes, 1984a), and changes in species composition (Newing et al, 1984; Removal of feral goats does not guarantee a Coblentz and van Vuren, 1987). Feral goats have return to former habitat conditions. Some conse- posed a major threat to over 26 per cent of island quences of eradication may even be undesirable. species in The IUCN Plant Red Data Book (Lucas Goat control on Round Island was accompanied and Synge, 1978). Every specimen of Ranunculus by a sudden increase in Tylophora taevigata, an aggressive creeper, which may restrain re-estab- *Daly and Goriup, 1987. lishment of native plants (Bullock and North, Eradication of feral goats 71

Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.93, on 02 Oct 2021 at 12:04:41, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605300022729 1985). Goat exclosures may show whether this almost always ineffective. Hunters need to be kind of problem is likely. highly skilled and committed to seeing the cull It is vital to consider the socio-economic implica- through to the last goat. Good team-work is tions of eradication. On Masafuera eradication of important; von Berg (1981) describes an exam- feral goats would merely force islanders to in- ple of lack of co-ordination within a hunting party crease their domestic stock, causing the same or resulting in few animals being shot, the rest being worse damage (I. Castro, in litt.). It is also impor- scared away. tant to remember that feral herds may be of sci- A good map and/or aerial photographs, com- entific value. bined with first-hand knowledge of the terrain are essential, particularly for identifying areas Having decided whether eradication is desirable where surviving goats may take refuge. Knowl- for a given island, the next step is to assess edge of locations of traditional sleeping sites and whether it is feasible. Attempts to eradicate or home range boundaries is helpful. In several control feral goat populations have been made campaigns (including Raoul Island and Santa on over 37 islands in the past 140 years. Eradi- Catalina) goats did not disperse rapidly from cation has been achieved on at least 20 of these traditional home ranges even under intense and the New Zealand Forest Service (NZFS) hunting pressure. Hunters can obviously exploit was involved in over half of these successful this phenomenon. Coblentz (1977) describes an campaigns. area of Santa Catalina that was cleared of goats Failure to appreciate the high potential repro- and not re-occupied for 20 years. However, he ductive capacity of feral goats has contributed to cautions that this phenomenon of slow dispersal the ineffectiveness of many campaigns. Before is enhanced by topography and may not be as control efforts on Pinta, the goat population in- pronounced in flat open terrain (B. E. Coblentz, creased from three to over 20,000 in just 12 in litt.). More information is needed on the effects years (Weber, 1971; Hamann, 1975). It is vital of control campaigns on goat dispersal. that the kill rate is greater than the rate of population growth. One way of measuring the success New Zealand Forest Service hunters often work of a campaign is to plot the number of goats re- with indicating dogs at the start of a campaign. moved against a measure of effort, such as These silent dogs remain in close contact with number of fieldworkers each day. This 'effort' is hunters and lead them to the goats. Taking dogs then increased until the daily cull shows a into conservation areas involves risk to native marked and continuously decreasing trend. wildlife, particularly if the dogs escape. It is Although circumstances will vary from island to essential that they are strictly controlled, es- island, it is advisable to maintain at least this level pecially in the vicinity of vulnerable species— of effort throughout the campaign. such as ground-nesting birds—and that they are neutered. Once culling becomes effective in reducing Aside from small hunting parties on foot, two population size, each kill becomes progressively other methods have been used to kill large num- more difficult. The final stage in the Raoul Island bers of goats rapidly: shooting from helicopters campaign illustrates this. The vegetation began and driving. The former technique reduced the to regenerate making survivors harder to locate goat population on San Clemente by around 91 and increasing food availability. The goats be- per cent, but the survivors were left in difficult ter- came extremely wary (Parkes, 1984b). In view of rain (Howard and Marsh, 1984; Laycock, 1984). such problems J. P. Parkes (pers. comm.) recom- The campaign was interrupted several times by mended that the aim should be 'to kill as many legal action, making it hard to assess how difficult (goats) as quickly and quietly as possible'. In it would have been to achieve eradication. Infor- order to do this campaigns commonly rely on mation from other campaigns suggests that it hunters on foot, with or without dogs. could have proved difficult. In Hawaii goats Casual ad hoc shooting—whether by local quickly learned to associate helicopters with people (Galapagos), weather men (Raoul Island) danger and responded to the sound by hiding or or off-duty naval personnel (Kahoolawe)—is remaining motionless (Anon., 1983). More infor- 72 Oryx Vol 23 No 2, April 1989

mation is needed, but in view of the above, have all been used at some time against verte- helicopters may be best reserved for the final brate pests, but all have serious drawbacks and stages of a campaign or as an occasional supple- no major advantages in comparison with com- ment to other techniques. pound 1080. Teams of people moving in well-co-ordinated Originally developed in the 1930s as a roden- lines have been used successfully in Hawaii to ticide, compound 1080 is currently used world- drive goats into the range of marksmen (Stone wide in vertebrate pest control. It is commonly and Keith, 1984). Driving can also be used in applied to artificial baits, which are either hand- conjunction with live-trapping, using temporary laid or dropped from the air. In recent years trials fences or natural barriers to funnel goats into cor- have been carried out using natural vegetation rals. One of the main disadvantages of driving baits, with some success against goats (Parkes, techniques is the high level of disturbance—both 1983). The method involves coating leaves on to the goats and other species. The goats' in- heavily goat-browsed trees with compound creasing wariness and decreasing numbers 1080 gel and bending them within reach of the makes driving less efficient over time. goats. Towards the end of a campaign this There is little doubt that large numbers of goats technique becomes inefficient as falling goat numbers allow vegetation regeneration. There could be killed rapidly by an intensive poisoning were no records of non-target poisoning in these campaign. The main problem is the effects on trials. native species. No poison kills goats exclusively and the only one worth considering is compound Kills of non-target birds and mammals are quite 1080, sodium fluoroacetate. Cyanide, arsenic, common in campaigns involving artificial baits strychnine, phosphorus and anti-coagulants (Batcheler, 1978). The risk of non-target kills is Eradication of feral goats 73

Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.93, on 02 Oct 2021 at 12:04:41, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605300022729 particularly high if bait is distributed from the air in. These techniques cannot be recommended in because control over bait location is minimal. the vicinity of vulnerable species. There is no clear evidence of long-term effects Helicopters can be very valuable for locating sur- on non-target populations in any compound vivors in difficult terrain and were successfully 1080 campaign, although this is not proof that no used in this way towards the end of the Raoul such effects exist. Impact will be particularly hard Island campaign (J. P. Parkes, pers. comm.). to measure on rare/vulnerable species (Spurr, 1979). An assessment of probable risk to non- The 'judas' goat technique relies on the sociable target species—particularly rare/vulnerable behaviour of goats. A neutered goat is fitted with ones—is important if a poisoning campaign is a radio-collar, released and tracked until it joins a contemplated. Such assessments should include herd. A helicopter can then be used to drive the identification of species likely to eat baits or goat goats towards hunters (Stone, 1984). The range carcasses and/or be highly susceptible to com- over which signals can be received depends on pound 1080. It is clearly unacceptable to con- the terrain, vegetation density and transmitter duct laboratory trials on endangered species, strength. The technique is intensive in skilled and although the susceptibility of some rare labour, equipment, time and money. species can be estimated from laboratory trials on similar but less endangered species, this is not Snares were used with some success by NZFS on possible for all species. (Mcllroy, in press). Raoul Island, but were only set on steep ledges and other places where death was likely to be Non-target species may be affected by sub-lethal rapid (J. P. Parkes, pers. comm.). Problems with doses of compound 1080. In many species such snaring include the unacceptable levels of suffer- effects include lethargy, which in reptiles may ing inflicted upon snared individuals and the lead to severe disruption of normal behavioural potential threat to non-target species. thermoregulation (Mcllroy et a/., 1985). The final stage of a campaign should be a follow- Studies from New Zealand have produced no up survey to check that no female goats have sur- evidence that compound 1080 persists for long vived. In the absence of other large mammals, periods in the environment (Batcheler, 1978). aerial survey with an infra-red video camera Such information is lacking for other parts of the could help locate survivors. world. In any eradication campaign it is essential to maintain morale, commitment and funding to Chemosterilants have attracted the attention of the end, and to appreciate from the start that vertebrate pest controllers in recent years, eradication of the last few goats can be extremely although at present there is no available chemo- expensive and difficult. On Raoul Island, eradica- sterilant suitable for goat eradication pro- tion of the last five goats cost US$6000 a goat (J. grammes. Future developments should be P. Parkes, pers. comm.). As experience from so monitored. many campaigns illustrates, 'if you do not eradi- In any campaign a point comes when most of the cate the last reproductive unit, you are wasting population has been killed. The few goats that your time and money' (P. Munton, in litt). survive are very wary and locating them is difficult. At this stage different techniques and Acknowledgments tactics are required. Potentially appropriate The original study on which this article is based was funded by the FFPS Oryx 100% Fund and the ICBP Conservation techniques include hunting with bailing dogs, Fund. I would like to thank Paul Goriup for his support and helicopter shooting platforms, use of 'judas' assistance. I am most grateful for the assistance and advice goats and kill snaring. kindly given by all my correspondents, in particular B. D. Bell (Wildlife Service, New Zealand), B. E. Coblentz (Department Bailing dogs bark as they chase goats. The NZFS of Fisheries and Wildlife, Oregon State University), W. E. employs such dogs to run goats down and hold Howard (Department of Wildlife and Fisheries, University of California), Sir C. Lever, J. C. Mcllroy (CSIRO, Australia), P. them until hunters arrive. Alternatively, they can Munton, The Nature Conservancy of Hawaii, J. P. Parkes be trained to bark in a particular way when (Forest Research Institute, New Zealand), M. R. Rudge goats are exhausted, allowing hunters to move (DSIR, Ecology Division, New Zealand). Finally, I would like 74 Oryx Vol 23 No 2, April 1989

Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.93, on 02 Oct 2021 at 12:04:41, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605300022729 to thank Clive Hambler for his encouragement and his Mammals: Problems and Potential (ed. P. Munton), pp. typewriter. 31-41, SSC, IUCN, Gland. Laycock, B. 1984. Goat tales. IUCN SSC Newsletter, 4, 4. References Leathwick, J.R., Hay, J.R. and Fitzgerald, A.E. 1983. The influence of browsing by introduced mammals on the Anon. 1983. Feral Goat Control. Unpubl. Rep. by Hawaii decline of North Island Kokako. N. Z. J. Ecol., 6,55-70. National Park Service. Atkinson, I.A.E. 1964. Relations between feral goats and Lucas, G. and Synge, H. 1978. The IUCN Plant Red Data vegetation in New Zealand. Proc. N. Z. Ecol. Soc, 11, Book. IUCN, Gland. 39-44. MacFarland, C.G., Villa, J. and Toro, B. 1974. Galapagos Giant Tortoise Geochelone elephantopus II: conservation Baker, J.K. and Reeser, D.W. 1972. Goat management prob- methods. Biol. Cons., 6, 198-212. lems in Hawaii Volcanoes National Park: a history, analysis and management plan. Natural Resources Re- Mcllroy, J.C., King, D.R. and Oliver, A.J. 1985. The sensitiv- port No. 2, US National Park Service. ity of Australian animals to 1080 poison, VIII. Amphibians and reptiles. Aust. WildlifeRes., 12,113-118. Batcheler, C.L. 1978. Compound 1080, its properties, effec- Mcllroy, J.C. (in press). The sensitivity of Australian animals tiveness, dangers and use. Unpubl. Rep. to the Minister of to 1080 poison, IX. Comparisons between the major Forests and Minister of Agriculture and Fisheries by N. Z. groups of animals and the potential danger to non-target Forest Service. species from 1080-poisoning campaigns. Aust. wild!. von Berg, F. 1981. Concise report on Goatex 1980. Ann. Research. Rep. of Charles Darwin Res. Centre, Santa Cruz, Mueller-Dombois, D. and Spatz, G. 1975. The influence of Galapagos Islands. feral goats on the lowland vegetation in Hawaii Vol- Bourne, W.R.P. 1975. Mammals on islands. New Scientist, canoes National Park. Phytocoenologia, 3, 1-29. 67,422-425. Newing, T.J., Daly, K.H. and Hambler, K. 1984. Feral goats Brooke, C. 1984. An historical-geographic perspective of the on Aldabra. Final Rep. on Southampton Univ. Exp. to feral Caprinae problem. In Feral Mammals: Problems and Aldabra, 1982. Unpubl. Potential (ed. P. Munton), pp. 17-30. SSC, IUCN, Gland. Parkes, J.P. 1983. Control of feral goats by poisoning with Bullock, D. 1977. Round Island—a tale of destruction. Oryx, Compound 1080 on natural vegetation baits and by 14,51-58. shooting. N. Z. J. Forestry Sci., 13, 266-274. Bullock, D. and North, S. 1985. Round Island in 1982. Oryx, Parkes, J.P. 1984a. Feral goats on Raoul Island, II. Diet and 18,36-41. notes on the flora. N. Z. J. Ecol., 7, 95-101. Calvopina, L. 1985. Biology, population structure and con- Parkes, J.P. 1984b. Policy and practice of feral goat control trol of feral goats on Santiago Island. Unpubl. Rep. to in New Zealand forests—the Raoul Island example. Charles Darwin Res. Centre, Santa Cruz, Galapagos Unpubl. Rep. to For. Res. Inst., N. Z. Forest Service. Islands. 87 pp. Pickard, J. 1976. The effect of feral goats on the vegetation of Coblentz, B.E. 1977. Some range relationships of feral goats Lord Howe Island. Aust. J. Ecol., 1,103-114. on Santa Catalina Island, California. J. Range Manage- Spurr, E. B. 1979. A theoretical assessment of the ability of ment, 30,415-419. bird species to recover from an imposed reduction in Coblentz, B.E. 1978. The effects of feral goats {Capra hircus) numbers, with particular reference to 1080 poisoning. N. on the Santa Catalina Island ecosystem. Biol. Cons., 13, Z.J.Ecol, 2, 46-63. 279-286. Stone, C. P. 1984. National Park Service research and man- Coblentz, B.E. and van Vuren, D. 1987. Biology and Control agement to control introduced animals in Hawaii. Proc. of Feral Goats on Aldabra Atoll, Republic of the Seychel- Ann. For. Wild!. Conf., May 10-12, pp. 68-80. les, Unpubl. Rep. to Seychelle Islands Foundation, Mahe, Stone, C.P. and Keith, J.O. 1984. Control of feral ungulates Republic of the Seychelles. and small mammals in Hawaii's National Parks: research Daly, K.H. and Goriup, P. 1987. Eradication of Feral Goats and management strategies. Unpubl. rep. from Small Islands. ICBP Study Report No. 17. Sykes, W.R. 1969. The effects of goats on the vegetation of Fullager, P.J. and Disney, H.J.S. 1975. The birds of Lord the Kermadec Islands. Proc. N. Z. Ecol. Soc, 16,13-16. Howe Island: a report on the rare and endangered Wardle, J., Hayward, J. and Herbert, J. 1973. Influence of species. XIIButt. ICBP, pp. 187-202. ungulates on the forests and scrublands of South West- Hamann, O. 1975. Vegetational changes in the Galapagos land. N. Z. J. For. Sci., 3,3-36. Islands during the period 1966-1973. Biol. Cons., 7, Weber, D. 1971. Pinta, Galapagos: Une ile a sauver. Biol. 37-59. Cons., 4, 8-12. Holdgate, M.W. 1967. The influence of introduced species Yates, S. 1984. On the cutting edge of extinction. Audubon, on the ecosystems of temperate oceanic islands. Proc. 86,62-85. and Papers of Tenth Technical Meeting, Lucerne, No. 9, ptIII, pp. 151-176, IUCN, Gland. Howard, W.E. and Marsh, R.E. 1984. Ecological implication K. Daly, 96 Caerphilly Road, Senghenydd, Caerphilly, CF8 and management of feral mammals in California. In Feral 2FW, UK.

Eradication of feral goats 75